Abrasive wheel and process of making it



Feb. 22, 194-4. Q clELL 2,342,121

ABRASIVE WHEELS AND PROCESS OF MAKING THEM Filed April 3, 1941 Patented Feb. 22, 1944 UNITED STATES PATENT OFFICE ABRASIVE WHEEL AND PROCESS OF KING IT This invention relates to abrasive articles such as grinding wheels. Serious objections to commercial grinding wheels are that they burn the material-whether metal or woodbeing ground, that their grinding faces become loaded with the material being ground, thereby impairing grinding efficiency, and that, in order to ameliorate, as far as possible, these objectionable conditions and particularly the burning of the material, the grinding operation must proceed l slowly, often with stoppages to allow the material being ground to be plunged into a cooling fluid to reduce the high temperature which it attainsafter a very short grinding period.

I have succeeded in producing grinding wheels entirely free of the foregoing objections. They are adapted for grinding steel, aluminum, brass, and in fact all metals, as well as wood, hard rubber and practically all materials that it might conceivably be desired to subject to a grinding operation. The successful production of such grinding wheels apparently depends upon both their composition and the method of manufacture. In other words, composition and method are apparently interdependent, since a grinding wheel of a similar composition produced by a different method of manufacture, or a grinding wheel of a substantially diflferent composition produced by the same method of manufacture, will not-certainly to a com- 80 parable degree-possess the advantages characteristic of my invention.

A grinding wheel embodying my invention comprises any known abrasive material, a permanent binder of the character hereinafter de- 5 scribed, and a temporary binder whose minimum necessary percentage is almost negligible; the finished wheel having certain physical characteristics, namely, high porosity and low speciflc gravity, as hereinafter more particularly de- 0 scribed, due to the composition of the original mix and the method of manufacture.

In the original mix the abrasive material may I be any of the materials commonly used in the manufacture of grinding wheels. selected will depend upon the use for which the grinding wheel is intended. Aluminum oxide and silica carbide are examples of such abrasive materials. They constitute, usually, the major part of the composition.

The constituents that are added to form the permanent and temporary binders may, and usually will, vary with the work that the grinding wheel is intended to-perform, and, for the same reason, those constituents which appear to The material 45 be essential will vary greatly in percentage. All of the following ingredients are added to the original mix in the manufacture of most of my improved grinding wheels, althohgh certain of them may be omitted inthe manufacture of grinding wheels intended to operate on certain materials, and the proportions of such ingredients will vary within the ranges specified (all the proportions being based on 100% abrasive material):

Any suitable clay, such as china clay (silica, S102), saddler clay, OK clay, puerine clay, and Albany slip clay: 1 to 25%.

Whiting, or any other form of calcium carbonate (08.00:), or other equivalent material, such as magnesium silicate: 1 to 20%.

Gypsum: 1 to 25%.

Any carbonaceous material, preferably lignite: .5 to 10%.

Borax: .06 to 2.5%.

Feldstar or flint: .06 to 2.5%.

Zinc oxide: .06 to .7

Shellac: .05 to .1%.

While in actual manufacture it is customary and more convenient to base the percentages of the added constituents upon 100% abrasive material, the original mix, based on the foregoing, may be said to comprise abrasive material 53 to clay 1 to 20%, whiting 1 to 16%, gypsum l to 20%, lignite .5 to 8%, borax .05 to 2%, feldspar or flint .05 to 2%, zinc oxide .05 to .5%, shellac .05 to .1%, the total percentage of material other than the abrasive material, however, of course, being that required to make up the A typical composition would thus contain abrasive material about 66%, clay 12%, whiting 8%, gypsum 8%, lignite 4%, borax 1%, feldstar or flint 1% and a small fraction of one per cent. each of zinc oxide and shellac.

In the manufacture of some wheels embodying my invention the feldspar and zinc oxide may be omitted. The more important, if not necessary, ingredients appear to be whiting, gypsum and lignite, or suitable equivalents, that is, materials having, as applied to the manufacture of grinding wheels, the properties of these ingredients. These ingredients are specified by way of example and not by way of necessary limitation, since the process of manufacture, as applied to a moulded dry mixture, is novel independently of the materials employed in the original mix except so far as the materials employed are necessary to produce, if said method is followed, the peculiar physical characteristics and operative functions hereinafter described.

It should also be explained that the added ingredients are not all existent in the final product, since one or more of them are entirely consumed in the baking operation or chemically react with other ingredients to form compounds not present in the cold mix. Thus, when lignite is added, it is apparently entirely consumed, and in such consumption contributes to the formation of the large pores which are a characteristic and necessary feature of the finished grinding wheel. It is, of course, known to add carbon in various forms to grinding wheel mixes in order to produce porosity in the finished wheel, but such materials are usually such as to leave voids comparable in volume to the volume of the combustible material itself. Apparently, in my composition, the added carbonaceous material, by its combustion, merely aids in the combustion of one or more of the other destructible materials, whose combustion is effective to produce voids whose volume is multiple times that of the originally added solids. Thus, in the baking operation, it is likely that the whiting (calcium carbonate, CaCOa) splits into lime (CaO) and carbon dioxide (CO2), the lime reacting with the sand (silica, SiOz) to form CaSiOa (calcium silicate) while the carbon dioxide expands with rising temperature and distorts or bloats the ceramic article. Other ingredients may contribute to produce this eflect. The constituents of the original mix producing the permanent binder may therefore be more accurately defined as a binder-forming and porosity-inducing material.

As a temporary binder I prefer to use shellac in a proportion varying from one-twentieth to onenow be described. The components are mixed dry and deposited in moulds. It is an essential feature of the process that'the mixture should be deposited in the moulds in a substantially dry state, preferably in a wholly dry state, and that it should not be subjected to substantial pressure either before or after being run into the moulds. Heretofore, in the manufacture of grinding wheels having compositions of abrasive material and a clay bond and which are handled dry, the mixture has been subjected to a pressure adequate to almost double the density that it otherwise would have. In the manufacture of my improved grinding wheels the use of substantial pressure in the forming would result in the production, after baking, of articles devoid of the advantages of my improved abrasive article or possessing them only in very minor degree. I am aware that in the manufacture of certain grinding wheels, such as puddle wheels, the dry constituents, which may be, for example, abrasive grains, clay, and feldspar or flint, are mixed with a very considerable percentage of water or water solutions of other ingredients and are then moulded, as distinguished from press-formed. The mixture is a fluid mud or plastic which is run into the moulds, as distinguished from the mixture, embodying my invention, which is deposited on the moulds in a substantially dry condition and then leveled off by means of any convenient tool.

The addition of water, as in the manufacture of puddle wheels, to the composition which I employ, before or after introduction to the moulds, would fail to produce a grinding wheel having the qualities which are detailed herein.

The moulded wheels are removed from the moulds and placed in a kiln, wherein they are subjected to a temperature of from 1700 to 2500 F.- usually from 2000 to 2400 F.--for a period, varying from 6 to 36 hours, dependent on the temperature, the composition and the size of the wheels.

Due to the dry, unpressed character of the moulded article, it is of a necessarily porous character before baking, but such pores are of a high degree of fineness and of practically uniform distribution and have no similarity whatever to the pores or voids produced as a result of the baking operation. In that operation the hot gases, produced as herein described and entrapped in closed pores, expand as the temperature becomes high and, due in part at least to the low specific gravity of the moulded article, produce coarse pores of irregular size and non-uniform distribution apd heterogeneous arrangement and of a total volume many times the total volume of the combustible ingredients of the original mix. The pressure exerted by the expanding trapped gases during the baking operation ultimately forces the gases to the outer surfaces of the casting, where they escape. During the baking operation those gases which are formed nearer these surfaces first escape, forming near these surfaces the large and irregularly distributed pores above described, through which previously trapped gases formed in the interior of the mass ultimately travel and escape until the described characteristic porosity is produced throughout the entire structure. The precise chemical changes and physical phenomena that occur are not, however, in all respects certain, but an adequate or accurate explanation thereof is of no importance, since the practice of the process with the illustrative starting ingredients described will produce an abrasive wheel having the described characteristics. Due to the substantially or entirely non-pressed character of the moulded and baked article, the density of the final product is very low, varying from .7 to 1.2

dependent upon the composition and the proportions of the ingredients. The average weight of a grinding wheel embodying my invention is but little over half that of ordinary commercial wheels of the same size form-pressed from dry mixtures. This low specific gravity is due to the described high porosity, the total volume of the pores usually approximating, or even exceeding, 50% of the total volume of the wheel, although the total weight of the porosity-inducing materials in the original mix is always a very small percentage of the entire mass,

From the foregoing description it may be understood that the degree of porosity of the finished abrasive article depends, first, upon the percentage of the initial binder and second upon the composition of such binder. With a given composition of binder, the greater the proportion of the binder, the greater the porosity. With a given proportion of binder, the larger the percentage of porosity-inducing materials, the greater the porosity. Therefore, the degree of porosity can be controlled within wide limits by varying the first described factors.

When the material to be ground, assuming it to be steel, is held with great pressure against a grinding wheel rotating at the usual high velocity, the material is ground away with remarkable speed without development of a high degree of heat in the material being ground, which, at the conclusion of the grinding operation, no matter how long and continued the operation may be, presents a clean, bright, and comparatively cool surface. So also, in grinding wood, there is no burning or singeing. At the conclussion of the drying operation the grinding surface is not loaded with material ground from the article operated upon and does not thereby have its grinding surface impaired.

The described behavoir of the abrading wheel may be accounted for by the fact that, during the rotation of the wheel, there is a continuous travel of atmospheric air through the interior of the wheel which escapes at the grinding surface, such air currents functioning as a continuously applied cooling agent which prevents any serious rise in the temperature of the surface of the article being ground. While a microscopic examination of cross-sections of the abrading article fails to reveal any other arrangement of pores than a heterogeneous one, experiments indicate that the travel of the air is mainly, if not wholly, in the case. for example, of a disc wheel,

' in an approximately radial direction, and in the case of awheel of any'shape, in the direction of its greater extension. That this is so is demonstrable "by plunging a disc wheel in water, which very rapidly fills the pores of the wheel, withdrawing the wheel and then holding it up with its fiat sides horizontal. In that position no water escapes. But if the wheel is held with its fiat sides vertical, water pours freely out of the downwardly facing peripheral edge and comparatively little time is required to drain it completely. Apparently the centrifugal force created by the rotation of the grinding wheel at high velocity is not a sole br complete explanation of the air-cooling quality of the wheel, since, independently thereof, there appears to be much more free intercommunication between the pores in the specified direction than in a direction perpendicular thereto.

It will be understood that I claim no novelty in a porous wheel, the advantages of which have been long recognized. In fact, all press-formed grinding wheels are more or less porous, but in none of them known to me is the porosity of such a character as to accomplish the new and useful results hereinbefore described. In pressformed grinding wheels there is apparently no free travel of air through the body of the wheel, or no such travel directed to the point of grinding, or no such travel directed to the point of grinding in sufiicient volume to counteract to any substantial degree the heat generated by the grinding action.

It is believed that the degree or character of the porosity in the finished wheel is not substantially affected, and is certainly not mainly governed, by the fineness of the grains of abrasive material, which may vary from the finest to the coarsest mesh commonly used in the production of grinding wheels.

It is dlfiicult and indeed impracticable to illustrate my improved wheel, since it differs little in appearance from ordinary press formed wheels of commerce, the only observable difference being a somewhat rougher exterior.

In the drawing:

Fig. 1 is a perspective view of the mould filled with the mixture in a dry state with the top leveled oil, without pressure, to give the mixture its final shape. Fig. 2 is a perspective view of the finished wheel. Fig. 3 is a cross-sectional view through the finished wheel, greatly enlarged to 5 show the characteristic porosity of the wheel.

The described mixture b is placed in the mould a and after setting is removed and baked, c showing the finished grinding wheel. The irregularly distributed black portions of d in Fig. 3 indil0 sible to illustrate them. It should be explained that the upper rough edge of Fig. 3 represents a very small arc'of the periphery of the wheel, while the area below same is a very small section of a section of the wheel adjacent thereto.

It will be understood that grinding wheels emboclying my invention may have any desired shape including all shapes in which abrasive wheels are usually manufactured.

What I claim and desire to protect by Letters Patent is:

1. A grinding wheel which comprises the baked product of a substantially unpressed dry mixture comprising 53 to 80% abrasive material, 1 to clay, I to 16% whiting, 1 to 25% gypsum and .5 to 10% combustible carbonaceous material. said wheel having a sponge-like structure, the pores of which are of varying size, irregular shape and heterogeneous distribution and in such free intercommunication as to allow free travel of a fluid therethrough toward the point of grinding when the grinding wheel is in use, thereby continuously cooling the ground face of the material being ground and substantially preventing load- 85 ing of the grinding face of the wheel with particles of said material.

2. A process of making abrasive grinding wheels which comprises mixing a major proportion of an abrasive material with a minor proportion of a binder-forming and porosity-inducing mixture that includes a combustible material and a material adapted at a high temperature to be transformed and produce expansible gases, pouring the mixture while in a substantially dry state into a mould in quantity sufficient without substantial pressure to impart to the wheel substantially its final contour and size and then, without substantial compression of the dry moulded mixture, baking the same at such temperature and for such time as to burn out said combustible material and cause said transformable material to effect bonding of the mass and sintering the whole into an integral mass and also to evolve gases which, with the gases of combustion produced by said combustible material, produce a sponge-like wheel having pores of varying size, irregular shape and heterogeneous distribution whose total volume greatly exceeds that of the total volume of those constituents in the unbaked original dry unpressed mix that are convertible into gases as above specified.

3. A process of making abrasive grinding wheels which comprises mixing, with an abrasive material, a permanent binder-forming and porosityinducing mixture that comprises clay, combustible carbonaceous material, gypsum and whiting, pouring the mixture while in a substantially dry state into a mould in quantity sufiicient, without substantial pressure, to impart to the mixture a size 70 and contour substantially corresponding to the size and contour of the finished wheel, and then, without substantial pressure of the moulded dry mixture, baking the same at the temperature and time required to so act upon the whiting 78 and clay as to effect a bonding of the mass, burn cate the pores of the wheel so far as it is pos-" out the combustible material and sinter the whole into an integral sponge-like mass having pores of varying size, irregular shape and heterogeneous distribution whose total volume greatly exceeds the total volume of the combustible material in the original unbalged mix.

4. The process of making abrasive grinding wheels which comprises mixing a major proportion of an abrasive material with a minor proportion of a binder-forming and porosity-inducing material one constituent of which is silica and another constituent of which comprises a combustile carbonaceous material and a mineral compound adapted at high temperature to react with the silica and to form a compound functionin: as a permanent binder and sinter the whole into an integral mass and also to evolve gases which, with the gases formed by the combustion of said carbonaceous material, form pores of varying size, irregular shape and heterogeneous distribution whose total volume greatly exceeds the volume ,of the constituents in the said dry mix before conversion into gases as above specifled, moulding the mixture substantially dry without substantial pressure, and baking the dry unpressed mixture at the temperature specified.

AUGUST 0. CELL. 

